This work was financially supported by Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The authors also acknowledge Journal of Materials Chemistry A (a Royal Society of Chemistry journal) for originally publishing this research (<a href="http://pubs.rsc.org/en/content/articlehtml/2016/ta/c6ta01177b">http://pubs.rsc.org/en/content/articlehtml/2016/ta/c6ta01177b</a>). PNNL is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830.

Note: All the solution preparations, cyclic voltammetry (CV) tests, and flow cell assembly and tests were carried out in an argon-filled glove box with water and O2 levels less than 1 ppm.
1. Electrochemical Evaluations of PTIO Flow Cells
<ol>
	<li>CV Test
	<ol>
		<li>Polish a glassy carbon electrode with 0.05 &#181;m gamma alumina powder, flush it with deionized water, put it in under vacuum at room temperature for overnight, and transfer it into a glove box.</li>
		<li>Dissolve silver nitrate (8.5 mg) with MeCN (5 mL) in the glove box, i.e., 10 mM AgNO3. Add the solution into the glass tube of a silver/silver nitrate reference electrode.</li>
		<li>Assemble the glassy carbon working electrode, a graphite felt strip counter electrode, and the silver/silver nitrate reference electrode on a 25 mL three-neck pear-shaped flask.</li>
		<li>Dissolve PTIO (52 mg) and tetrabutylammonium hexafluorophosphate (TBAPF6, 0.87 g) in MeCN (1.10 g), i.e., 0.1 M PTIO/1.0 M TBAPF6. Add the solution to the flask to submerge the tips of the three electrodes.</li>
		<li>Connect the electrodes to an electrochemical workstation. Measure the CV curves within the voltage range of -1.75-0.75 V at a scan rate of 100 mV/s. Determine the theoretical cell voltage of the PTIO flow battery by the potential gap between the two redox pairs. 
		NOTE: It should be noted that the Ag/Ag+ reference electrode configuration is a pseudo-reference electrode by its nature. As a result, redox peaks might shift during long-term CV measurements. Nevertheless, such shift normally has a negligent influence on the voltage gap between redox couples, and would not affect the cell voltage value.</li>
	</ol>
	</li>
	<li>Flow Cell Assembly
	<ol>
		<li>Cut the graphite felts to an area of 1 x 10 cm2 using a razor blade. Similarly, cut a porous separator to an area of 3 x 12 cm2.</li>
		<li>Dry the flow battery parts (cell compartments, tubing, 5 mL glass vials, graphite felts, and a porous separator) in a vacuum oven at 70 &#176;C for overnight, move them into the glove box, and cool down to the environmental temperature.</li>
		<li>Assemble the flow cell parts in the order of an end plate, a copper plate current collector, a half cell, a graphite felt, a gasket, a porous separator, a graphite felt, a half cell, a copper plate current collector, and an end plate. Fasten the assembly with eight threaded bolts against the two end plates using a torque wrench pre-set at 125 inch pounds. Connect the electrolyte flow tubings to the flow cell. The cell assembly is shown in Figure 1.</li>
	</ol>
	</li>
	<li>Demonstration of Symmetric Electrochemistry
	<ol>
		<li>Assemble the flow cell according to Section 1.2. Dissolve PTIO (10 mg) and TBAPF6 (3.3 g) with MeCN (4.4 g) in the glove box, i.e., 5.0 mM PTIO/1.0 M TBAPF6. Add 4 mL of the solution to each of the two glass vials. Pump the electrolytes to flow using a peristaltic pump at a flow rate of 20 mL/min.</li>
		<li>Connect the positive and negative current collectors of the flow cell to the battery tester. Charge the flow cell at a constant current density of 5 mA/cm2 until the voltage reached 1.9 V. Stop the charging. Pump out the electrolytes into the glass vials.</li>
		<li>Mix a 1 mL positive electrolyte with a 1 mL negative electrolyte in a separate vial. Now there are four electrolytes: the original, the positive, the negative, and the mixed.</li>
		<li>Measure electron spin resonance (ESR) spectrum of the above four electrolytes.25
		<ol>
			<li>With tube sealant, seal a small amount (~10 &#181;L) of the positive and negative in a PTFE tubing (1/16&#34; OD and 1/32&#34; ID) at both ends, and then seal it in a quartz ESR tube (4 mm diameter).</li>
			<li>Mount the ESR tubing to an ESR spectrometer fitted with an SHQE resonator with microwave frequency ~9.85 GHz (X band).</li>
			<li>Collect the ESR spectrum for the four electrolytes in Section 1.3.3.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Flow Cell Test
	<ol>
		<li>Assemble a flow cell following Section 1.2.</li>
		<li>Dissolve PTIO (1.05 g) and TBAPF6 (3.50 g) with MeCN (3.60 g) in the glove box, i.e., 0.5 M PTIO/1.0 M TBAPF6. Add 4 mL of the solution to each glass vial. Flow the electrolytes at 20 mL/min.</li>
		<li>Connect the positive and negative current collectors of the flow cell to an electrochemical workstation. Measure the impedance of the flow cell in the frequency range from 100 kHz to 1 Hz at the open circuit potential. Calculate the area-specific resistivity (ASR) by multiplying the ohmic resistance (high-frequency impedance) by the active area of the flow cell.</li>
		<li>Connect the positive and negative current collectors of the flow cell to the battery tester. Set up the voltage cutoffs of 0.8 and 2.2 V and the constant current of 20 mA cm-2 in the battery operation software. Repeatedly charge/discharge the PTIO flow cell.</li>
	</ol>
	</li>
</ol>
2. FTIR-based SOC Determination
<ol>
	<li>FTIR Feasibility Validation
	<ol>
		<li>Prepare the following three electrolyte solutions in the glove box: (a) MeCN (0.50 g); (b) TBAPF6 (0.23 g) with MeCN (0.30 g), i.e., 1.0 M TBAPF6; (c) PTIO (75 mg) and TBAPF6 (0.25 g) with MeCN (0.26 g), i.e., 0.5 M PTIO/1.0 M TBAPF6.</li>
		<li>Measure FTIR for the three electrolyte solutions.
		<ol>
			<li>Add a small volume (~0.05 mL) of each solution to a sealable FTIR cell with KBr windows and a path length of 0.2 mm. Seal the FTIR cell.</li>
			<li>Put the FTIR cell into a storage container and transfer it out of the glove box.</li>
			<li>Quickly mount the FTIR cell to a spectrometer and collect the FTIR spectrum.</li>
		</ol>
		</li>
		<li>Assemble a flow cell following Section 1.2.</li>
		<li>Dissolve PTIO (1.05 g) and TBAPF6 (3.50 g) with MeCN (3.60 g) in the glove box, i.e., 0.5 M PTIO/1.0 M TBAPF6. Add 4 mL of the solution to each glass vial. Flow the electrolytes at 20 mL/min.</li>
		<li>Fully charge the flow cell until the voltage reaches 2.2 V. Stop the charging and the pump.</li>
		<li>Measure the FTIR spectra for both the positive and negative electrolytes, respectively, following the procedure in Section 2.1.2.</li>
		<li>Prepare a series of PTIO solutions (0.05-0.5 M) in 1.0 M TBAPF6 in MeCN in the glove box with the compositions in Table 1.</li>
		<li>Measure the FTIR spectrum for each of the solutions in Section 2.1.6, following the procedure in Section 2.1.2.</li>
	</ol>
	</li>
	<li>FTIR Measurement of SOC
	<ol>
		<li>Assemble a flow cell following Section 1.2.</li>
		<li>Dissolve PTIO (2.9 g) and TBAPF6 (9.6 g) with MeCN (9.8 g) in the glove box, i.e., 0.5 M PTIO/1.0 M TBAPF6. Add 11 mL of the solution to each of the two glass vials. Flow the electrolytes at 20 mL/min.</li>
		<li>Charge the flow cell at a constant current of 10 mA/cm2 at a flow rate of 20 mL/min.</li>
		<li>At the charge time of 0, 18, 36, 54, and 72 min, stop the cell charge and electrolyte flow, take small aliquots (0.2 mL) of the electrolytes from anolyte and catholyte side glass vials, and then resume the cell.</li>
		<li>Measure the FTIR spectrum for the above five sample aliquots, following the procedure in Section 2.1.2.</li>
		<li>Measure the ESR spectrum for the above five sample aliquots, following the procedure in Section 1.3.4.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Dunn, B., Kamath, H., Tarascon, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrical+Energy+Storage+for+the+Grid:+A+Battery+of+Choices.">Electrical Energy Storage for the Grid: A Battery of Choices.</a> Science. 334 (6058), 928-935 (2011).</li><li>Yang, Z. G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+Energy+Storage+for+Green+Grid.">Electrochemical Energy Storage for Green Grid.</a> Chem. Rev. 111 (5), 3577-3613 (2011).</li><li>Wang, W., Luo, Q., Li, B., Wei, X., Li, L., Yang, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+Progress+in+Redox+Flow+Battery+Research+and+Development.">Recent Progress in Redox Flow Battery Research and Development.</a> Adv. Funct. Mater. 23 (8), 970-986 (2013).</li><li>Skyllas-Kazacos, M., Chakrabarti, M. H., Hajimolana, S. A., Mjalli, F. S., Saleem, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Progress+in+Flow+Battery+Research+and+Development.">Progress in Flow Battery Research and Development.</a> J. Electrochem. Soc. 158 (5), 55-79 (2011).</li><li>Weber, A. Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Redox+Flow+Batteries:+A+Review.">Redox Flow Batteries: A Review.</a> J. Appl. Electrochem. 41 (10), 1137-1164 (2011).</li><li>Noack, J., Roznyatovskaya, N., Herr, T., Fischer, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Chemistry+of+Redox-Flow+Batteries.">The Chemistry of Redox-Flow Batteries.</a> Angew. Chem. Int. Ed. 54 (34), 9775-9808 (2015).</li><li>Soloveichik, G. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flow+Batteries:+Current+Status+and+Trends.">Flow Batteries: Current Status and Trends.</a> Chem. Rev. 115 (20), 11533-11558 (2015).</li><li>Leung, P., Li, X., de Leon, C. P., Berlouis, L., Low, C. T. J., Walsh, F. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Progress+in+Redox+Flow+Batteries,+Remaining+Challenges+and+Their+Applications+in+Energy+Storage.">Progress in Redox Flow Batteries, Remaining Challenges and Their Applications in Energy Storage.</a> RSC Adv. 2 (27), 10125-10156 (2012).</li><li>Gong, K., Fang, Q., Gu, S., Li, S., Yan, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonaqueous+Redox-Flow+Batteries:+Organic+Solvents,+Supporting+Electrolytes,+and+Redox+Pairs.">Nonaqueous Redox-Flow Batteries: Organic Solvents, Supporting Electrolytes, and Redox Pairs.</a> Energy Environ. Sci. 8 (12), 3515-3530 (2015).</li><li>Shin, S. H., Yun, S. H., Moon, S. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Review+of+Current+Developments+in+Non-aqueous+Redox+Flow+Batteries:+Characterization+of+Their+Membranes+for+Design+Perspective.">A Review of Current Developments in Non-aqueous Redox Flow Batteries: Characterization of Their Membranes for Design Perspective.</a> RSC Adv. 3 (24), 9095-9116 (2013).</li><li>Cappillino, P. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Application+of+Redox+Non-Innocent+Ligands+to+Non-Aqueous+Flow+Battery+Electrolytes.">Application of Redox Non-Innocent Ligands to Non-Aqueous Flow Battery Electrolytes.</a> Adv. Energy Mater. 4 (1), 1300566 (2014).</li><li>Suttil, J. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metal+Acetylacetonate+Complexes+for+High+Energy+Density+Non-aqueous+Redox+Flow+Batteries.">Metal Acetylacetonate Complexes for High Energy Density Non-aqueous Redox Flow Batteries.</a> J. Mater. Chem. A. 3 (15), 7929-7938 (2015).</li><li>Brushett, F. R., Vaughey, J. T., Jansen, A. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+All-Organic+Non-aqueous+Lithium-Ion+Redox+Flow+Battery.">An All-Organic Non-aqueous Lithium-Ion Redox Flow Battery.</a> Adv. Energy Mater. 2 (11), 1390-1396 (2012).</li><li>Wei, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Radical+Compatibility+with+Nonaqueous+Electrolytes+and+Its+Impact+on+an+All-Organic+Redox+Flow+Battery.">Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery.</a> Angew. Chem. Int. Ed. 54 (30), 8684-8687 (2015).</li><li>Nagarjuna, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+of+Redox-Active+Polymer+Molecular+Weight+on+the+Electrochemical+Properties+and+Transport+Across+Porous+Separators+in+Nonaqueous+Solvents.">Impact of Redox-Active Polymer Molecular Weight on the Electrochemical Properties and Transport Across Porous Separators in Nonaqueous Solvents.</a> J. Am. Chem. Soc. 136 (46), 16309-16316 (2014).</li><li>Wei, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=TEMPO-Based+Catholyte+for+High-Energy+Density+Nonaqueous+Redox+Flow+Batteries.">TEMPO-Based Catholyte for High-Energy Density Nonaqueous Redox Flow Batteries.</a> Adv. Mater. 26 (45), 7649-7653 (2014).</li><li>Wei, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Towards+High-Performance+Nonaqueous+Redox+Flow+Electrolyte+Via+Ionic+Modification+of+Active+Species.">Towards High-Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species.</a> Adv. Energy Mater. 5 (1), 1400678 (2015).</li><li>Fan, F. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polysulfide+Flow+Batteries+Enabled+by+Percolating+Nanoscale+Conductor+Networks.">Polysulfide Flow Batteries Enabled by Percolating Nanoscale Conductor Networks.</a> Nano Lett. 14 (4), 2210-2218 (2014).</li><li>Pan, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+Way+Toward+Understanding+Solution+Chemistry+of+Lithium+Polysulfides+for+High+Energy+Li-S+Redox+Flow+Batteries.">On the Way Toward Understanding Solution Chemistry of Lithium Polysulfides for High Energy Li-S Redox Flow Batteries.</a> Adv. Energy Mater. 5 (16), 1500113 (2015).</li><li>Escalante-Garcia, I. L., Wainright, J. S., Thompson, L. T., Savinell, R. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Performance+of+a+Non-Aqueous+Vanadium+Acetylacetonate+Prototype+Redox+Flow+Battery:+Examination+of+Separators+and+Capacity+Decay.">Performance of a Non-Aqueous Vanadium Acetylacetonate Prototype Redox Flow Battery: Examination of Separators and Capacity Decay.</a> J. Electrochem. Soc. 162 (3), 363-372 (2015).</li><li>Wei, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microporous+Separators+for+Fe/V+Redox+Flow+Batteries.">Microporous Separators for Fe/V Redox Flow Batteries.</a> J. Power Sources. 218, 39-45 (2012).</li><li>Skyllas-Kazacos, M., Kazacos, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=State+of+Charge+Monitoring+Methods+for+Vanadium+Redox+Flow+Battery+Control.">State of Charge Monitoring Methods for Vanadium Redox Flow Battery Control.</a> J. Power Sources. 196 (20), 8822-8827 (2011).</li><li>Brooker, R. P., Bell, C. J., Bonville, L. J., Kunz, H. R., Fenton, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determining+Vanadium+Concentrations+Using+the+UV-Vis+Response+Method.">Determining Vanadium Concentrations Using the UV-Vis Response Method.</a> J. Electrochem. Soc. 162 (4), 608-613 (2015).</li><li>Petchsingh, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectroscopic+Measurement+of+State+of+Charge+in+Vanadium+Flow+Batteries+with+an+Analytical+Model+of+VIV-VV+Absorbance.">Spectroscopic Measurement of State of Charge in Vanadium Flow Batteries with an Analytical Model of VIV-VV Absorbance.</a> J. Electrochem. Soc. 163 (1), 5068-5083 (2016).</li><li>Duan, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Symmetric+Organic-Based+Nonaqueous+Redox+Flow+Battery+and+Its+State+of+Charge+Diagnostics+by+FTIR.">A Symmetric Organic-Based Nonaqueous Redox Flow Battery and Its State of Charge Diagnostics by FTIR.</a> J. Mater. Chem. A. 4 (15), 5448-5456 (2016).</li><li>Potash, R. A., McKone, J. R., Conte, S., Abru&#241;a, H. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+Benefits+of+a+Symmetric+Redox+Flow+Battery.">On the Benefits of a Symmetric Redox Flow Battery.</a> J. Electrochem. Soc. 163 (3), 338-344 (2016).</li><li>Kim, H. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Tetradentate+Ni(II)+Complex+Cation+as+a+Single+Redox+Couple+for+Non-aqueous+Flow+Batteries.">A Tetradentate Ni(II) Complex Cation as a Single Redox Couple for Non-aqueous Flow Batteries.</a> J. Power Sources. 283, 300-304 (2015).</li><li>Shinkle, A. A., Sleightholme, A. E. S., Griffith, L. D., Thompson, L. T., Monroe, C. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Degradation+Mechanisms+in+The+Non-aqueous+Vanadium+Acetylacetonate+Redox+Flow+Battery.">Degradation Mechanisms in The Non-aqueous Vanadium Acetylacetonate Redox Flow Battery.</a> J. Power Sources. 206, 490-496 (2012).</li><li>Li, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+Properties+of+an+All-Organic+Redox+Flow+Battery+Using+2,2,6,6-Tetramethyl-1-Piperidinyloxy+and+N-Methylphthalimide.">Electrochemical Properties of an All-Organic Redox Flow Battery Using 2,2,6,6-Tetramethyl-1-Piperidinyloxy and N-Methylphthalimide.</a> Electrochem. Solid-State Lett. 14 (12), 171-173 (2011).</li><li>Schaltin, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Towards+an+All-Copper+Redox+Flow+Battery+Based+on+a+Copper-Containing+Ionic+Liquid.">Towards an All-Copper Redox Flow Battery Based on a Copper-Containing Ionic Liquid.</a> Chem. Commun. 52, 414-417 (2016).</li><li>Luo, Q., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capacity+Decay+and+Remediation+of+Nafion-based+All-Vanadium+Redox+Flow+Batteries.">Capacity Decay and Remediation of Nafion-based All-Vanadium Redox Flow Batteries.</a> ChemSusChem. 6 (2), 268-274 (2013).</li></ol>

The authors have nothing to disclose.

As we demonstrated before,25 FTIR is capable of non-invasively detecting the SOC of the PTIO flow battery. As a diagnostic tool, FTIR is particularly advantageous because of its easy accessibility, fast response, low cost, small space requirement, facility for online incorporation, no detector saturation, and the ability to correlate structural information to investigate molecular evolutions during flow battery operation. Figure 3e illustrates a proposed flow battery device integr...

Redox flow batteries store energy in liquid electrolytes that are contained in external reservoirs and are pumped to internal electrodes to complete electrochemical reactions. The stored energy and power can thus be decoupled leading to excellent design flexibility, scalability, and modularity. These advantages make flow batteries well-suited for stationary energy storage applications for integrating clean yet intermittent renewable energies, increasing grid asset utilization and efficiency, and improving energy resiliency and security.1,2,3 Traditional aqueous flow batteries suffer from limited energy density, mostly due to the narrow voltage window to avoid water electrolysis.4,5,6,7,8 In contrast, non-aqueous electrolytes based flow batteries are being widely pursued because of the potential for achieving high cell voltage and high energy density.9,10 In these efforts, a variety of flow battery chemistries have been investigated, including metal-coordination complexes,11,12 all-organic,13,14 redox active polymers,15 and lithium hybrid flow systems.16,17,18,19
However, the potential of non-aqueous flow batteries has yet to be fully demonstrated due to the major technical bottleneck of limited demonstration under flow battery-relevant conditions. This bottleneck is closely associated with a number of performance-limiting factors. First, the small solubility of most electroactive materials leads to low energy density delivery by non-aqueous flow cells. Second, the rate capability of non-aqueous flow batteries is largely limited by the high electrolyte viscosity and resistivity at relevant redox concentrations. The third factor is the lack of high-performance membranes. Nafion and ceramic membranes show low ionic conductivity with non-aqueous electrolytes. Porous separators have demonstrated decent flow cell performance, but suffer considerable self-discharge because of relatively large pore size.14,20 Typically, mixed-reactant electrolytes containing both anolyte and catholyte redox materials (1:1 ratio) are used to reduce redox materials crossover, which however sacrifices the effective redox concentrations, typically by half.14,21 Overcoming the aforementioned bottleneck requires improvements in materials discovery, battery chemistry design, and flow cell architecture to achieve battery-relevant cycling.
Battery status monitoring is essentially important for reliable operations. Off-normal conditions including overcharge, gas evolution, and material degradation can cause damages to battery performance and even battery failure. Especially for large-scale flow batteries involving large amounts of battery materials, these factors can cause serious safety issues and investment loss. State of charge (SOC) describing the depth of charge or discharge of flow batteries is one of the most important battery status parameters. Timely SOC monitoring can detect potential risks before they reach threatening levels. However, this area seems to be under-addressed so far, especially in non-aqueous flow batteries. Spectrophotoscopic methods such as ultraviolet-visible (UV-vis) spectroscopy and electrolyte conductivity measurements have been evaluated in aqueous flow battery for SOC determination.22,23,24
We have recently introduced a novel symmetric non-aqueous flow battery design based on a new ambipolar redox material, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO).25 This flow battery holds the promise to address the aforementioned challenges of non-aqueous flow batteries. First, PTIO has a high solubility (2.6 M) in the battery solvent of acetonitrile (MeCN) that is promising to enable a high energy density. Second, PTIO exhibits two reversible redox pairs that are moderately separated and thus can form a symmetric battery chemistry by itself. We have also demonstrated that a distinguishable PTIO peak in the FTIR spectra can be correlated with the concentration of unreacted PTIO in the flow cell, which leads to spectroscopic determination of the SOC, as cross-validated by ESR results.26 Here we present a protocol to elaborate procedures for electrochemical evaluations and FTIR-based SOC diagnostics of the PTIO symmetric flow battery. This work is expected to trigger more insights in maintaining the safety and reliability during long-term flow battery operations, especially in real-world grid applications.

<table><tbody><tr><td>PTIO</td><td>TCI America</td><td>A5440</td><td>&gt;98.0%</td></tr><tr><td>Tetrabutylammonium hexafluorophosphate</td><td>Sigma-Aldrich</td><td>86879</td><td>electrochemical grade, &amp;ge;99.0%</td></tr><tr><td>MeCN</td><td>BASF</td><td>50325685</td><td>Battery grade</td></tr><tr><td>Silver nitrate</td><td>Sigma-Aldrich</td><td>204390</td><td>99.9999% trace metals basis</td></tr><tr><td>Gamma alumina powder</td><td>CH Instruments</td><td>CHI120</td><td></td></tr><tr><td>Graphite felt</td><td>SGL</td><td>GFD3</td><td>Vacuum-dry at 70 &amp;deg;C for 24 h</td></tr><tr><td>Porous separator</td><td>Daramic</td><td>AA800</td><td>Vacuum-dry at 70 &amp;deg;C for 24 h</td></tr><tr><td>Battery Tester</td><td>Wuhan LAND electronics Co., Ltd.</td><td>Lanhe</td><td>1 A current range</td></tr><tr><td>Electrochemical Workstation</td><td>Solartron Analytical</td><td>ModuLab</td><td></td></tr><tr><td>glove box</td><td>MBRAUN</td><td>Labmaster SP</td><td>oxygen and water levels &amp;lt;1 ppm</td></tr><tr><td>ESR spectrometer</td><td>Bruker&amp;nbsp;</td><td>Elexsys 580&amp;nbsp;</td><td>Equipped with an SHQE resonator with microwave frequency ~9.85 GHz (X band) at 2 mW power, with 100 kHz field modulation</td></tr></tbody></table>

a protocol for electrochemical evaluations and state of charge diagnostics of a symmetric organic redox flow battery

Redox flow batteries have been considered as one of the most promising stationary energy storage solutions for improving the reliability of the power grid and deployment of renewable energy technologies. Among the many flow battery chemistries, non-aqueous flow batteries have the potential to achieve high energy density because of the broad voltage windows of non-aqueous electrolytes. However, significant technical hurdles exist currently limiting non-aqueous flow batteries to demonstrate their full potential, such as low redox concentrations, low operating currents, under-explored battery status monitoring, etc. In an attempt to address these limitations, we recently reported a non-aqueous flow battery based on a highly soluble, redox-active organic nitronyl nitroxide radical compound, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). This redox material exhibits an ambipolar electrochemical property, and therefore can serve as both anolyte and catholyte redox materials to form a symmetric flow battery chemistry. Moreover, we demonstrated that Fourier transform infrared (FTIR) spectroscopy could measure the PTIO concentrations during the PTIO flow battery cycling and offer reasonably accurate detection of the battery state of charge (SOC), as cross-validated by electron spin resonance (ESR) measurements. Herein we present a video protocol for the electrochemical evaluation and SOC diagnosis of the PTIO symmetric flow battery. With a detailed description, we experimentally demonstrated the route to achieve such purposes. This protocol aims to spark more interests and insights on the safety and reliability in the field of non-aqueous redox flow batteries.

The unique advantages of the symmetric PTIO flow battery system are highly ascribed to the electrochemical properties of PTIO, an organic nitroxide radical compound. PTIO can undergo electrochemical disproportionation reactions to form PTIO+ and PTIO&#8722; (Figure 2a). These two redox pairs are moderately separated by a voltage gap of ~1.7 V (Figure 2b) and can be used as both anolyte and catholyte redox materials in a symmetric bat...

Watch this Scientific Journal Video about A Protocol for Electrochemical Evaluations and State of Charge Diagnostics of a Symmetric Organic Redox Flow Battery at JoVE.com

A Protocol for Electrochemical Evaluations and State of Charge Diagnostics of a Symmetric Organic Redox Flow Battery

We present the protocols for electrochemically evaluating a symmetric non-aqueous organic redox flow battery and for diagnosing its state of charge using FTIR.

Redox flow batteries have been considered as one of the most promising stationary energy storage solutions for improving the reliability of the power grid ...

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Research

JoVE Journal

Chemistry

10.4K Views.  Joint Center for Energy Storage Research (JCESR). We present the protocols for electrochemically evaluating a symmetric non-aqueous organic redox flow battery and for diagnosing its state of charge using FTIR.

Video: A Protocol for Electrochemical Evaluations and State of Charge Diagnostics of a Symmetric Organic Redox Flow Battery

Protocol of Electrochemical Test and Characterization of Aprotic Li-O2 Battery

We demonstrate a method for electrochemical testing of an aprotic Li-O2 battery. An aprotic Li-O2 battery is made of a Li-metal anode, an aprotic electrolyte, and an O2-breathing cathode. The aprotic electrolyte is a solution of lithium salt with aprotic solvent; and porous carbon is commonly used as the cathode substrate. To improve the performance, an electrocatalyst is deposited onto the porous carbon substrate by certain deposition methods, such as atomic layer deposition (ALD) and wet-chemistry reaction. The as-prepared cathode materials are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray absorption near edge structure (XANES). A Swagelok-type cell, sealed in a glass chamber filled with pure O2, is used for the electrochemical test on a battery test system. The cells are tested under either capacity-controlled mode or voltage controlled mode. The reaction products are investigated by electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and Raman spectroscopy to study the possible pathway of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). This protocol demonstrates a systematic and efficient arrangement of routine tests of the aprotic Li-O2 battery, including the electrochemical test and characterization of battery materials.

Protocol of Electrochemical Test and Characterization of Aprotic Li-O2 Battery

A protocol for the electrochemical testing of an aprotic Li-O2 battery with the preparation of electrodes and electrolytes and an introduction of the frequently used methods of characterization is presented here.

We demonstrate a method for electrochemical testing of an aprotic Li-O2 battery. An aprotic Li-O2 battery is made of a Li-metal anode, an aprotic ...

Engineering

Electrochemical Impedance Spectroscopy as a Tool for Electrochemical Rate Constant Estimation

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). In the case of fast reversible electrochemical processes, current is predominantly affected by the rate of diffusion, which is the slowest and limiting stage. EIS is a powerful technique that allows separate analysis of stages of charge transfer that have different AC frequency response. The capability of the method was used to extract the value of charge transfer resistance, which characterizes the rate of charge exchange on the electrode-solution interface. The application of this technique is broad, from biochemistry up to organic electronics. In this work, we are presenting the method of analysis of organic compounds for optoelectronic applications.

Electrochemical impedance spectroscopy (EIS) of species that undergo reversible oxidation or reduction in solution was used for determination of rate constants of oxidation or reduction.

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). ...

Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds

Cyclic voltammetry (CV) is a technique used in the analysis of organic compounds. When this technique is combined with electron paramagnetic resonance (EPR) or ultraviolet-visible and near-infrared (UV-Vis-NIR) spectroscopies, we obtain useful information such as electron affinity, ionization potential, band-gap energies, the type of charge carriers, and degradation information that can be used to synthesize stable organic electronic devices. In this study, we present electrochemical and spectroelectrochemical methods to analyze the processes occurring in active layers of an organic device as well as the generated charge carriers.

In this article, we describe electrochemical, electron paramagnetic resonance, and ultraviolet-visible and near-infrared spectroelectrochemical methods to analyze organic compounds for application in organic electronics.

Cyclic voltammetry (CV) is a technique used in the analysis of organic compounds. When this technique is combined with electron paramagnetic resonance ...

In Situ Lithiated Reference Electrode: Four Electrode Design for In-operando Impedance Spectroscopy

Extending operating voltage of Li-ion batteries results in higher energy output from these devices. High voltages, however, may trigger or accelerate multiple processes responsible for long-term performance decay. Given the complexity of physical processes occurring inside the cell, it is often challenging to achieve a full understanding of the root causes of this performance degradation. This difficulty arises in part from the fact that any electrochemical measurement of a battery will return the combined contributions of all components in the cell. Incorporation of a reference electrode can solve part of the problem, as it allows the electrochemical reactions of the cathode and the anode to be individually probed. A variation in the voltage range experienced by the cathode, for example, can indicate alterations in the pool of cyclable lithium ions in the full-cell. The structural evolution of the many interphases existing in the battery can also be monitored, by measuring the contributions of each electrode to the overall cell impedance. Such wealth of information amplifies the reach of diagnostic analysis in Li-ion batteries and provides valuable input to the optimization of individual cell components. In this work, we introduce the design of a test cell able to accommodate multiple reference electrodes, and present reference electrodes that are appropriate for each specific type of measurement, detailing the assembly process in order to maximize the accuracy of the experimental results.

The incorporation of reference electrodes in a Li-ion battery provides valuable information to elucidate degradation mechanisms at high voltages. In this article, we present a cell design that accommodates multiple reference electrodes, along with the assembly steps to assure maximum accuracy of the data obtained in electrochemical measurements.

Extending operating voltage of Li-ion batteries results in higher energy output from these devices. High voltages, however, may trigger or accelerate ...

Three-electrode Coin Cell Preparation and Electrodeposition Analytics for Lithium-ion Batteries

As lithium-ion batteries find use in high energy and power applications, such as in electric and hybrid-electric vehicles, monitoring the degradation and subsequent safety issues becomes increasingly important. In a Li-ion cell setup, the voltage measurement across the positive and negative terminals inherently includes the effect of the cathode and anode which are coupled and sum to the total cell performance. Accordingly, the ability to monitor the degradation aspects associated with a specific electrode is extremely difficult because the electrodes are fundamentally coupled. A three-electrode setup can overcome this problem. By introducing a third (reference) electrode, the influence of each electrode can be decoupled, and the electrochemical properties can be measured independently. The reference electrode (RE) must have a stable potential that can then be calibrated against a known reference, for example, lithium metal. The three-electrode cell can be used to run electrochemical tests such as cycling, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). Three-electrode cell EIS measurements can elucidate the contribution of individual electrode impedance to the full cell. In addition, monitoring the anode potential allows the detection of electrodeposition due to lithium plating, which can cause safety concerns. This is especially important for the fast charging of Li-ion batteries in electric vehicles. In order to monitor and characterize the safety and degradation aspects of an electrochemical cell, a three-electrode setup can prove invaluable. This paper aims to provide a guide to constructing a three-electrode coin cell setup using the 2032-coin cell architecture, which is easy to produce, reliable, and cost-effective.

Three-electrode cells are useful in studying the electrochemistry of lithium-ion batteries. Such an electrochemical setup allows the phenomena associated with the cathode and anode to be decoupled and examined independently. Here, we present a guide for construction and use of a three-electrode coin cell with emphasis on lithium plating analytics.

As lithium-ion batteries find use in high energy and power applications, such as in electric and hybrid-electric vehicles, monitoring the degradation and ...

Extending the Lifespan of Soluble Lead Flow Batteries with a Sodium Acetate Additive

In this report, we present a method for the construction of a soluble lead flow battery (SLFB) with an extended cycle life. By supplying an adequate amount of sodium acetate (NaOAc) to the electrolyte, a cycle life extension of over 50% is demonstrated for SLFBs via long-term galvanostatic charge/discharge experiments. A higher quality of the PbO2 electrodeposit at the positive electrode is quantitatively validated for NaOAc-added electrolyte by throwing index (TI) measurements. Images acquired by scanning electron microscopy (SEM) also exhibit more integrated PbO2 surface morphology when the SLFB is operated with the NaOAc-added electrolyte. This work indicates that electrolyte modification can be a plausible route to economically enable SLFBs for large-scale energy storage.

A protocol for the construction of a soluble lead flow battery with an extended lifespan, in which sodium acetate is supplied in the methanesulfonic electrolyte as an additive, is presented.

In this report, we present a method for the construction of a soluble lead flow battery (SLFB) with an extended cycle life. By supplying an adequate ...

Evaluating the Electrochemical Properties of Supercapacitors using the Three-Electrode System

The three-electrode system is a basic and general analytical platform for investigating the electrochemical performance and characteristics of energy storage systems at the material level. Supercapacitors are one of the most important emergent energy storage systems developed in the past decade. Here, the electrochemical performance of a supercapacitor was evaluated using a three-electrode system with a potentiostat device. The three-electrode system consisted of a working electrode (WE), reference electrode (RE), and counter electrode (CE). The WE is the electrode where the potential is controlled and the current is measured, and it is the target of research. The RE acts as a reference for measuring and controlling the potential of the system, and the CE is used to complete the closed circuit to enable electrochemical measurements. This system provides accurate analytical results for evaluating electrochemical parameters such as the specific capacitance, stability, and impedance through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Several experimental design protocols are proposed by controlling the parameter values of the sequence when using a three-electrode system with a potentiostat device to evaluate the electrochemical performance of supercapacitors. Through these protocols, the researcher can set up a three-electrode system to obtain reasonable electrochemical results for assessing the performance of supercapacitors.

The protocol describes the evaluation of various electrochemical properties of supercapacitors using a three-electrode system with a potentiostat device.

The three-electrode system is a basic and general analytical platform for investigating the electrochemical performance and characteristics of energy ...

Magnetometric Characterization of Redox-Active MOFs Solid-State Electrochemistry

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks  

Ex situ magnetic surveys can directly provide bulk and local information on a magnetic electrode to reveal its charge storage mechanism step by step. Herein, electron spin resonance (ESR) and magnetic susceptibility are demonstrated to monitor the evaluation of paramagnetic species and their concentration in a redox-active metal-organic framework (MOF).

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs ...

Chronoamperometry Analysis of EET in Lactiplantibacillus plantarum with DHNA

Characterizing Mediated Extracellular Electron Transfer in Lactic Acid Bacteria with a Three-Electrode, Two-Chamber Bioelectrochemical System

Many bacteria perform extracellular electron transfer (EET), whereby electrons are transferred from the cell to an extracellular terminal electron acceptor. This electron acceptor can be an electrode and electrons can be delivered indirectly via a redox-active mediator molecule. Here, we present a protocol to study mediated EET in Lactiplantibacillus plantarum, a probiotic lactic acid bacterium widely used in the food industry, using a bioelectrochemical system. We detail how to assemble a three-electrode, two-chambered bioelectrochemical system and provide guidance on characterizing EET in the presence of a soluble mediator using chronoamperometry and cyclic voltammetry techniques. We use representative data from 1,4-dihydroxy-2-naphthoic acid (DHNA)-mediated EET experiments with L. plantarum to demonstrate data analysis and interpretation. The techniques described in this protocol can open new opportunities for electro-fermentation and bioelectrocatalysis. Recent applications of this electrochemical technique with L. plantarum demonstrated an acceleration of metabolic flux towards producing fermentation end-products, which are critical flavor components in food fermentation. As such, this system has the potential to be further developed to alter flavors in food production or produce valuable chemicals.

Here we present a protocol for characterizing mediated extracellular electron transfer (EET) in lactic acid bacteria using a three-electrode, two-chamber bioelectrochemical system. We illustrate this method with Lactiplantibacillus plantarum and the redox mediator 1,4-dihydroxy-2-naphthoic acid and provide a thorough description of the electrochemical techniques used to evaluate mediated EET.

Many bacteria perform extracellular electron transfer (EET), whereby electrons are transferred from the cell to an extracellular terminal electron ...

Biochemistry

Electrochemical Measurements of Supported Catalysts Using a Potentiostat/Galvanostat

Source: Laboratory of Dr. Yuriy Rom&#225;n &#8212; Massachusetts Institute of Technology
A potentiostat/galvanostat (often referred to as simply a potentiostat) is an instrument that measures current at an applied potential (potentiostatic operation) or measures potential at an applied current (galvanostatic operation) (Figure 1). It is the most commonly used instrument in the electrochemical characterization of anode and cathode materials for fuel cells, electrolyzers, batteries, and supercapacitors.
Conventionally, these anode and cathode materials are interfaced with a potentiostat via a three-electrode electrochemical cell. The electrode leads from the potentiostat are connected to the reference electrode, the counter electrode (often called the auxiliary electrode), and the working electrode (which contains the test material of interest). The electrochemical cell is then filled with a high ionic strength electrolyte solution, such as an acidic, alkaline, or salt solution. The media for this high ionic strength solution is typically aqueous; however, for applications necessitating higher operating cell potential windows, such as batteries and supercapacitors, non-aqueous media is often used. The cell media is degassed with an inert gas (to prevent unwanted side reactions) or with a test gas (if the test reaction involves a gas at one of the electrodes).
Alternatively, a salt bridge or membrane is employed to maintain ionic contact if the two half cells are to be measured in different electrolytes. In heterogeneous electrocatalysis, this type of &#34;two compartment&#34; cell is often used if the test molecule at the working electrode is also reactive at the counterelectrode. This happens frequently as the counterelectrode typically employed is platinum, which is a highly active catalyst for many reactions. Here, single compartment cells will be used, where all three electrodes are in the same media.
This video will explain the process of polishing a working electrode, preparing a catalyst ink, mounting the catalyst ink onto the working electrode, preparing the electrochemical cell, and then performing electrochemical measurements. The measurements that are performed include: cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronopotentiometry (CP), and chronoamperometry (CA).
<img alt="Figure 1" src="/files/ftp_upload/5698/5698fig1.jpg" /> 
Figure 1. An example of a single compartment electrochemical cell. a.) Teflon cap, b.) glass cell, c.) Pt wire counter electrode, d.) working electrode, e.) Ag/AgCl reference electrode, f.) 0.5 M aqueous sulfuric acid electrolyte solution.